1. Field of the Invention
The present invention relates to a control technique for correcting a velocity variation of a rotating member, such as a photosensitive drum or a drive roller that drives a photosensitive drum or a transfer belt, of an image forming apparatus, such as a printer, copier or multifunction peripheral.
2. Description of the Related Art
In an image forming apparatus such as a copying machine or a printer employing an electrophotographic process, it is known that, if a rotational variation (velocity variation) occurs to a rotating member (such as a photosensitive drum serving as a photosensitive member) in an image forming unit, a drive unit or a driving force transmission mechanism of the rotating member, an output image becomes uneven (i.e., image quality is degraded).
In addition, color image forming apparatuses may employ a plurality of image forming units to form a toner image with various colors. Generally, the plurality of image forming units is arranged in a moving direction of a transfer material transport belt onto which the toner image is transferred from the photosensitive drum, or in a direction of an intermediate transfer belt onto which the toner image is primarily transferred.
In the following description, the transfer material transport belt and the intermediate transfer belt are collectively referred to “transfer belt”.
In the image forming apparatus of this type, a rotational variation may occur due to a mechanical misalignment such as eccentricity of a drive gear (driving force transmission mechanism) of the photosensitive drum and eccentricity of a drive motor shaft (drive unit) in the image forming unit. As a result, registrations in a sub-scanning direction of the toner images formed on the photosensitive drums may not be aligned on a transfer material onto which multiple images are finally transferred. Similarly, the registrations in the sub-scanning direction of the toner images may not be aligned due to a rotational variation caused by eccentricity of a drive gear of a transfer belt and eccentricity of a drive motor shaft (drive unit). (Hereinafter, the above phenomenon is referred to as color misregistration.) Accordingly, there is a problem that unevenness of the image or color misregistration may appear in the sub-scanning direction due to eccentricity or the like caused by a mechanical misalignment which can be generated during a process of manufacturing and assembling drive-system parts of the photosensitive drum or transfer belt. In particular, the eccentricity that appears in a reduction gear portion for driving force transmission or meshing pitch unevenness between gears, leads to a periodic angular velocity variation of the rotating member.
Therefore, in order to solve the problem, Japanese Patent Application Laid-open No. 2-43574 discusses a method in which a rotational motion of a rotating member is grasped by detecting an angular velocity using an equipment such as an encoder mounted on a shaft of the rotating member or alternatively, by detecting a surface velocity of a transfer belt. Thus, the periodic velocity variation caused by the eccentricity of the gear is detected and the drive can be corrected in accordance with a velocity instruction for eliminating the periodic velocity variation. Consequently, a velocity variation is canceled and a stable rotational motion of the rotating member can be obtained.
In Japanese Patent Application Laid-open No. 2-43574, it discusses a need to improve resolution with which a detecting unit performs detection in a comparatively high frequency band such as meshing pitch unevenness of gears. Japanese Patent Application Laid-open No. 2-43574 attempts to achieve highly precise drive control by improving resolution of a detection device.
However, in the configuration as described above, there is a problem that a drive unit has to minutely change velocity by performing correction control, and a drive torque associated with correction changes significantly when velocity is changed. Therefore, the drive motor can step out in a drive system employing a pulse motor. Even if no stepping out occurs, since the drive motor uses a wide frequency band, vibration can be applied and added to a resonance frequency in the driving force transmission unit or a machine which can induce a resonance phenomenon in a drive and mechanical system.
Therefore, in order to solve the problem described above, employing of a moving average process in response to detected rotational velocity information is discussed in Japanese Patent Application Laid-open No. 5-252774, Japanese Patent Application Laid-open Nos. 7-303385 and 10-066373. However, in the case where the moving average process is performed, a calculation error may occur due to the rounding-down or rounding-up of fractions in the calculation process. Namely, in the moving average process, since the moving average process is applied to a plurality of divided intervals in one revolution (one rotation of the rotating member), calculation errors can accumulate at each interval. Thus, an instruction of average velocity actually given to a drive system when the rotating member makes one revolution may be shifted by accumulated errors from a desired average velocity. In addition, the average velocity may also be shifted in a case where a phase of a rotational variation period of the drive unit or driving force transmission unit at the upstream of a photosensitive drum or a transfer belt, does not match one rotation of a rotational variation period of a roller for driving the photosensitive drum or the transfer belt. If this shift of the average velocity adversely affects drive of the photosensitive drum or transfer belt, an image magnification varies in a sub-scanning direction, which causes color misregistration in the sub-scanning direction in the case of a multi-transferring device.
Further, as illustrated in FIG. 7A, average moving values are calculated at each interval (at each dotted interval on “t” axis in the figure), and as illustrated in FIG. 7B, control is performed to give a predetermined correction velocity instruction (correction value) to the intervals. As a result, a control residual error occurs as illustrated in FIG. 7C. Rotational variation due to this control residual error appears as image unevenness.
In addition, similar to the average moving process for calculating the average moving value, there is a case in which filter calculation is performed in a unit that prevents a resonance phenomenon. However, also in this case, similar to the moving average process, an actual detection velocity has a variety of frequency bands even if a cut off frequency is strictly restricted when performing filter calculation. Accordingly, a difference may occur between a desired average velocity and an average velocity instruction obtained with a correction output value after filter calculation in one revolution of a rotating member. In other words, the velocity profile after filter calculation is shifted from a target average velocity, which causes image defect such as color misregistration or magnification variation similar to the above described methods.